1. Field of the Invention
The present invention relates to a polymer having a cinnamic acid moiety as a photosensitive group, a phase difference film using the polymer, and an optical film using the phase difference film, and also an alignment film using the polymer. More specifically, the invention relates to a photosensitive polymer having satisfactory solubility to an organic solvent and having liquid crystallinity, and use of the polymer allows preparation of a phase difference film having desired optical characteristics without needing a liquid crystal alignment film. The phase difference film is suitable for use in an optical application, particularly, an optical compensation film or a phase difference film in a liquid crystal display, a patterned phase difference film used for a passive glasses type 3D display, or the like. Furthermore, the photosensitive polymer can also align a liquid crystal material, and therefore is suitable for lamination of optical anisotropic materials such as a polymerizable liquid crystal and a liquid crystalline polymer.
2. Description of the Related Art
A liquid crystal display device is used in various kinds of liquid crystal display apparatuses, including a monitor of a notebook-sized personal computer or a desktop personal computer, a viewfinder of a video camera, a projection display and a television. The liquid crystal display device is further utilized as an optoelectronics-related device such as an optical printer head, an optical Fourier transformation device and a light valve. As a liquid crystal display device that has been applied so far, a display device using a nematic liquid crystal is predominantly applied, and a practical application has been made for a liquid crystal display device having a twisted nematic (TN) mode in which a direction of alignment of liquid crystals in the vicinity of one substrate, and a direction of alignment of liquid crystals in the vicinity of the other substrate are twisted at an angle of 90 degrees, a super twisted nematic (STN) mode in which the directions of alignment are ordinarily twisted at an angle of 180 degrees or more, and a so-called thin-film-transistor (TFT) mode in which a thin-film transistor is used.
However, a viewing angle at which an image can be properly visually recognized is narrow in the liquid crystal display devices, and when the image is viewed from an oblique direction, luminance or contrast may be occasionally decreased, and luminance inversion may be occasionally caused in a halftone. The issue of the viewing angle has been recently improved by a liquid crystal display device having a TN mode in which an optical compensation film is used, a multi-domain vertical alignment (MVA) mode in which a technology of homeotropic alignment and a technology of protrusion structure are simultaneously applied (see Patent literature No. 1), an in-plane switching (IPS) mode according to a transverse electric field mode (see Patent literature No. 2), or the like.
Development of technology on the liquid crystal display device has been achieved not only by an improvement of a driving mode and device structure as described above but also by an improvement of a member used for the display device. Among the members used for the display device, the optical compensation film or the phase difference film is one of important elements related to an image display quality for achieving an improvement in contrast or extension of a viewing angle range in the liquid crystal display device, and plays an increasingly important role with achieving a high quality of the display device year by year. As such an optical compensation film or a phase difference film, a stretched film having refractive index anisotropy or a film prepared by aligning and polymerizing a polymerizable liquid crystal compound is used.
With regard to the optical compensation film or the phase difference film, further precise control of refractive index anisotropy has been recently required for further enhancing an improvement in contrast or extension of the viewing angle range in the image display device. In such a present circumstance, the stretched film has a problem of a limited stretching direction and difficulty in precise control of the refractive index anisotropy during manufacture.
In the film prepared by aligning and polymerizing the polymerizable liquid crystal compound, the polymerizable liquid crystal compound shows optical anisotropy in a liquid crystal state, and polymerizes, thereby the alignment thereof being immobilized. Specific examples of alignment states in which the polymer is immobilized include “homogeneous” (homogeneous alignment), “tilt” (tilted alignment), “homeotropic” (homeotropic alignment) and “twist” (twisted alignment). Control of alignment of the polymerizable liquid crystal compounds allows precise control of refractive index anisotropy (see Patent literature Nos. 3 to 4).
Furthermore, a passive glasses type 3D display is put in practical use as one of 3D display types, and in the 3D display, a phase difference plate is mounted on a liquid crystal display panel. As the phase difference plate, study has been conducted for a patterned phase difference plate prepared by aligning a polymerizable liquid crystal compound to a liquid crystal alignment film subjected to alignment treatment by a photoalignment method.
However, the optical compensation film or the phase difference film formed of the polymerizable liquid crystal compound requires alignment of the polymerizable liquid crystal compound in order to develop desired optical characteristics. In general, the polymerizable liquid crystal compound is applied onto the liquid crystal alignment film subjected to alignment treatment to control alignment, and therefore the optical compensation film or the phase difference film has a problem that requires the liquid crystal alignment film.
A proposal has been recently made for a phase difference film in which a liquid crystalline polymer is irradiated with light to control molecular alignment (see Patent literature Nos. 5 to 9). The liquid crystalline polymer has a photosensitive group that reacts by irradiation with light, thereby allowing control of an alignment axis. Therefore, the phase difference film can be prepared without using an alignment film. Moreover, the phase difference film allows three-dimensional alignment achievement of which has been difficult by a conventional stretched film or polymerizable liquid crystal material (see Patent literature No. 7 or 8). In order to manufacture the phase difference film using such a polymer, the polymer is first dissolved into a solvent to apply the resulting solution onto a substrate, dry the resulting material, and irradiate the resulting film with linearly polarized light or the like, and heat the film, and thus the phase difference film is prepared. As the solvent for dissolving the polymer in such a process, a solvent having toxicity and environmental load as low as possible is desired. However, the arts have a problem of poor solubility of the polymer described in Patent literatures Nos. 5 to 8 and use of a solvent having comparatively high toxicity or environmental load. Furthermore, for a substrate of the optical compensation film or the phase difference film, a plastic such as polyethylene terephthalate (PET), triacetyl cellulose (TAC) and a cyclic olefinic polymer may be occasionally used. Such a plastic substrate has lower solvent resistance in comparison with glass, and therefore use of a solvent that is hard to allow erosion (dissolution, swelling) of the substrate is required.
The polymer described in Patent literature No. 9 has relatively good solubility, and has a possibility of solving the problem described above. However, when a phase difference film is prepared from the polymer, characteristics of the phase difference film, such as optical anisotropy may be occasionally insufficient without passing through a complicated manufacturing step, and desire is expressed for a material that can further simply and positively prepare a high-quality phase difference film.